Coal crushers have been available and in use for many years for breaking coal into small pieces suitable for handling and burning. The best crushers available are capable of breaking the coal into precisely the desired range of particle sizes which are best for the particular application such as pipeline transportation or particular coal preparation plants. The breaking of the coal into precisely desired sizes is done by passing the coal between two crusher rolls which have intermeshing teeth which are spaced apart a certain distance axially and circumferentially so that the coal is broken to the desired size, with minimal production of "fines."
After a period of use, the chisel teeth on the crusher rolls wear down to the extent that the coal pieces passing through the crusher exceed the desired size. When this happens, it is necessary to correct the situation either by building up the teeth on the crusher rolls by welding or by replacing the rolls altogether. In either case, it is necessary to remove the roll and replace it with a new or repaired roll while the worn roll is being repaired or replaced.
Removal of the roll from the coal crusher necessitates that the crusher be taken out of service while the rolls are being replaced. It is therefore desirable that the roll replacement process be as fast as possible to reduce the amount of time that the coal crusher is out of service. However, coal crusher rolls are extremely large and heavy, especially for large, efficient machines and the process of removing a roll is a lengthy one. It requires the disassembly and/or disconnection of the roll bearings and the drive mechanism and requires very careful and precise manipulation of the large heavy roll by a large crane. Because of the size and mass of the roll, the process of removing a roll must be done with great care to prevent damage to surrounding equipment, to the roll itself, and to the workers in the vicinity. The replacement of the roll is just as difficult and ticklish a business as the removal of the roll and the entire roll replacement process could take several days. In the time that this work is under way, the coal crusher is out of operation with resulting economic loss and loss of production.
It has been proposed in the past to make coal crushers with removable roll segments. The removable roll segments of the prior art have suffered from numerous disadvantages which have retarded their acceptance by the industry. One problem is the achieving of a close face fit between the roll segment and the roll on which it is mounted. Unless this face fit is extremely precise, the cyclic forces on the roll segment can cause it to fret and abrade the roll on which it is mounted and can also cause fatigue of the fasteners by which the segment is attached to the roll. The prior art has attempted to solve that problem by various strategems, such as zinc coating and precision machining, but these strategems are expensive and not always successful.
Because of the extreme vibration encountered in a coal crusher, the fasteners which hold the segments to the roll must be extremely resistant to becoming loose under vibration. This problem has been recognized in the past and has been approached by using lock nuts on the attachment bolts to lock the nut in place, by cotter pins through the end of the bolt or other similar techniques. These are undesirable because of the greatly increased time required to secure the fastener and also because, while these techniques prevent the attachment bolts from falling out of the machine altogether, they do not always prevent them from becoming loose. The loose bolts are useless to hold the segment in place and allow it to flex and fret against the support roll.
Prior art coal crushers with removable segments have utilized ribs or splines in the backside of the segment which fit into corresponding grooves running axially along the roll face to prevent circumferential forces exerted on the segments during crushing from shearing the attachment bolts. The cost of special machining in the support roll and corresponding difficulties in ensuring the correct placement and size of the ribs in the segment casting has greatly increased the cost of the removable segment system, but it or some variant of this scheme for carrying circumferential stresses has usually been considered necessary because the shearing of the attachment bolts on the segments can result in catastrophic failure of the crusher and require replacement of the entire crusher.